Recently, diagnoses and medical treatments using electronic endoscopes are commonly performed in the medical field. An electronic endoscope has a long insert section to be inserted into a patient's body cavity. The electronic endoscope incorporates an imaging device such as a CCD at its distal end. The electronic endoscope is connected to a light source apparatus. The light source apparatus delivers light to the electronic endoscope via a light guide such as an optical fiber. The electronic endoscope emits the light from its distal end to illuminate the body cavity. The imaging device at the distal end of the insert section captures an image of a tissue site in the body cavity while the body cavity is illuminated. The image is subjected to various processes in a processing apparatus connected to the electronic endoscope, and then displayed on a monitor. With the use of the electronic endoscope, a doctor can observe an image of the tissue site in the patient's body cavity real-time, which ensures accurate diagnosis.
The light source apparatus is a white light source, for example, a xenon lamp that emits white broadband light in a wavelength range from blue to red. An image (hereafter referred to as broadband image) captured with the illumination of the white broadband light shows the overall condition of the tissue site, for example, mucosa located on a surface of the tissue site, blood vessels inside the tissue, the shape and condition of the surface of the mucosa such as protrusion or depression, and the presence or absence of polyp.
In addition to the observation of the broadband image (normal visible image), observation with “special light”, that is, the observation using a narrow band light source that emits narrow band light in a specific wavelength range is performed as disclosed in Japanese Patent No. 3559755 corresponding to U.S. Patent No. 2003/0176768, for example. Light penetrates deeper into the tissue site as its wavelength increases, namely, blue light, green light, and red light increase in penetration depth in this order. Using the difference in the penetration depth, the endoscope system of Japanese Patent No. 3559755 generates a narrow band image in which blood vessels at a specific depth (for example, at the surface or at the depths) are enhanced. For example, with the emission of blue narrow band light having a small penetration depth, a narrow band image is generated with an enhanced surface blood vessel region. With the emission of green narrow band light having the penetration depth larger than that of the blue narrow band light, a narrow band image is generated with enhanced blood vessels located at the depth deeper than the surface. Thus, the blood vessels at a specific depth are more clearly observed in the narrow band image than in the broadband image.
Although the narrow band image of Japanese Patent No. 3559755 enhances the blood vessels, visibility of a region other than the blood vessels, such as the condition of mucosa, the shape of the mucosal surface, or the presence or absence of polyp, becomes significantly low when compared to the broadband image, because the narrow band image is a monochrome image using narrow band light of a single color. To observe the blood vessel region and the remaining region at a time, the narrow band image and the broadband image are displayed on the monitor side by side. The doctor needs to move his or her eyes between the narrow band image and the broadband image. Accordingly, the two images cannot be observed at a glance and cannot be easily contrasted with each other.
In the case where the region of interest is not the blood vessels, it becomes easy to observe the region of interest when the blood vessels are inconspicuous. The blood vessels are inconspicuous in the broadband image compared to those in the narrow band image. However, within the broadband image, the blood vessels, especially, the surface blood vessels are conspicuous compared to the remaining region, interfering with the observation of the remaining region. To solve this problem, visibility of the surface blood vessels needs to be reduced. For example, a pattern analysis may be performed to the broadband image to identify the position of the blood vessel region to reduce the contrast and visibility thereof.
However, the broadband image includes a considerable amount of information other than that of the blood vessels. The above method using the pattern analysis has poor accuracy because there is a high possibility that a region other than the blood vessels may be mistaken as the blood vessels.
Even if the blood vessel region is identified using the pattern analysis of the broadband image, the depth of the blood vessels cannot be determined, for example, the surface blood vessels cannot be distinguished from the deep blood vessels. Accordingly, reducing the contrast of one blood vessel region (for example, the surface blood vessel region) also reduces all the other blood vessel regions in the broadband image. The deep blood vessels are naturally inconspicuous compared to the surface blood vessels, so the need of the contrast reduction of the deep blood vessels is rare. If the contrast of the deep blood vessels is reduced, information essential to the deep blood vessels may be lost.